1. Field of the Invention.
This invention relates to an improved stabilizer and method for stabilizing 1,1,1-trichloroethane. More particularly, it relates to the use of free radical scavengers as stabilizing agents for trichloroethane. Most especially, it relates to a process for making semiconductor and related devices in which trichloroethane stabilized with a free radical scavenger is employed as a degreasing solvent in the process.
2. Description of the Prior Art.
Because of their highly desirable solvent properties, chlorinated hydrocarbon solvents are widely used in industrial processes. The one major drawback that these solvents have as a group is their instability. The decomposition of these solvents produces acids, is caused by the presence of light or air, and is catalyzed by such metals as iron, aluminum, magnesium, and their alloys. The presence of acid decomposition products further serves to increase the breakdown of these solvents. As pointed out in, for example, U.S. Pat. No. 3,787,509, the decomposition problem is particularly severe in the case of 1,1,1-trichloroethane. As pointed out in, for example, U.S. Pat. No. 2,621,215, mixtures of chlorinated solvents, in particular tetrachloroethylene and trichloroethane, tend to decompose at an accelerated rate, compared to tetrachloroethylene alone.
In order to overcome problems associated with the instability of chlorinated solvents, a wide variety of stabilizers for them are employed. For example, the above mentioned U.S. Pat. No. 3,787,509 lists a number of different types of compounds that have been found useful in stabilizing trichloroethane against decomposition. Specifically, commercially available trichloroethane obtained under the brand name of Chlorothene VG from the Dow Chemical Co., Midland, Mich., is stabilized by the addition of nitromethane, dioxane and acetonitrile. It is believed that these and similar stabilizers as listed in the U.S. Pat. No. 3,787,509 work as acid scavengers to remove the acid decomposition products of the chlorinated solvents, thus substantially reducing the rate of decomposition.
In the fabrication of integrated circuits and other semiconductor devices, as well as lead frames for them, and the like, freedom of the device surfaces from contamination is very important to assure proper operation of the finished devices. Chlorinated solvents, in particular, trichloroethylene, have achieved a high degree of acceptance as degreasing solvents for the semiconductor devices. In use, the semiconductor devices are immersed in vigorously boiling chlorinated solvents to degrease them. Such semiconductor devices usually include aluminum or other metallization which catalyzes the decomposition of the chlorinated solvents and is also subject to attack by the acid decomposition products of the chlorinated solvents. It thus may be recognized that the degreasing of semiconductor devices represents a difficult environment for use of chlorinated solvents, even if stabilized. Commercially available stabilized trichloroethylene has proved to be satisfactory for this application.
However, a major problem associated with continued use of trichloroethylene for this purpose is that the trichloroethylene molecule contains an unsaturated double bond. Chlorinated solvents containing unsaturated double bonds cannot be discharged into the atmosphere under present Environmental Protection Agency regulations. The ability to continue their use is therefore dependent on the ability to remove them completely from exhaust gases. Various other chlorinated solvents not containing unsaturated double bonds have therefore been considered as replacements for trichloroethylene, since they are not prohibited in exhaust gases. Initial attempts to use trichloroethane in this environment have not been successful. Even the commercially available stabilized Chlorothene VG will withstand decomposition during use described above as a degreasing solvent for semiconductor devices for only about 1 or 2 hours, far too short a time to allow its practical use as such a degreasing solvent.
Thus, while the art of stabilizing chlorinated solvents is a well developed one, there remains a need for a stabilized chlorinated solvent composition which will meet the stringent demands of degreasing semiconductor devices, yet which is acceptable under Environmental Protection Agency regulations.